On the Making the Web blog, there's this post that talks about looking forward with your code and making it ready for when PHP6 comes around.

If you want to make use of PHP 6 when it comes, you're going to have to write your new scripts so they are compatible, and possibly change some of your existing scripts. To start making your scripts PHP 6 compatible, I've compiled a list of tips to follow when scripting.

There's only five things in his list (like "stop using magic_quotes" and "don't register long arrays") but the comments provide many more additional gotchas to look out for and new features that will be included.